Necked housing in float zone refining

ABSTRACT

A METHOD OF PRODUCING A SEMI-CONDUCTOR SINGLE CRYSTAL BY FLOATING ZONE MELTING OF A VERTICAL ROD OF SEMI-CONDUCTOR MATERIAL IN A CONTROLLABLE ATMOSPHERE INSIDE A TUBULAR, HERMETICALLY CLOSED CHAMBER, WHICH ROD IS CAPABLE OF PERFORMING AT LEAST ONE TRANSLATORY MOVEMENT RELATIVE TO A COIL SURROUNDING THE ROD AND TRAVERSED BY HIGH-FREQUENCY CURRENT. THE COIL IS ARRANGED TO SURROUND THE CHAMBER AT THE LEVEL OF NARROWED PRION THEREOF. THE ROD IS MELTED FROM ITS LOWER END AND IS BROUGHT INTO CONTACT WITH A SEED CRYSTAL. THE SINGLE CRYSTAL FORMED HAS THE   SAME DIAMETER AS THE ROD. THE MOLTEN ZONE CONTAIN A NARROWED ZONE, THE DIAMETER OF WHICH IS SMALLER THAN THE DIAMETER OF THE SOLID PORTIONS OF THE ROD.

Sept. 12, 1972 M. AYEL 3,690,848

NECKED HOUSING IN FLOAT ZONE REFINING Filed July 21, 1970 Fig.1

INVENTOR.

BY MICHEL AYEL United States Patent Oflfic e U.S. Cl. 23-301 SP 7 Claims ABSTRACT OF THE DISCLOSURE A method of producing a semi-conductor single crystal by floating zone melting of a vertical rod of semi-conductor material in a controllable atmosphere inside a tubular, hermetically closed chamber, which rod is capable of performing at least one translatory movement relative to a coil surrounding the rod and traversed by high-frequency current. The coil is arranged to surround the chamber at the level of narrowed portion thereof. The rod is melted from its lower end and is brought into contact with a seed crystal. The single crystal formed has the same diameter as the rod. The molten zone contains a narrowed zone, the diameter of which is smaller than the diameter of the solid portions of the rod.

The invention relates to a method of producing a semiconductor single crystal by floating zone melting a vertical rod of semiconductor material in a controllable atmosphere inside a tubular, hermetically closed chamber, said rod being adapted to perform at least one translation movement relative to a coil surrounding the rod and traversed by a high-frequency current and furthermore to a semiconductor single crystal produced by said method and to a device for carrying out said method.

The method of producing single crystals by floating zone melting is generally employed for the manufacture of semiconductor devices. A commonly polycrystalline semiconductor rod is held in a vertical position, locally melted and brought into contact with a seed crystal, the melting zone shifting through the rod and the material being gradually recrystallized into a single crystal. The fusion is usually obtained by means of inductive currents by means of a coil which is substantially coaxial to the rod and is traversed by a high-frequency current. The melting zone is held in a state of equilibrium between the original rod and the seed crystal by the effect of its own weight and the surface tension of the liquid; therefore such a floating molten zone assumes a shape which is characterized by slight bulging in the lower portion and a neck-like contraction in the upper portion.

During this process the molten zone has to be protected from air and any impurities. For this purpose zone melting is carried out in a hermetically closed chamber having a controllable atmosphere or in vacuo.

The inductance coil may be arranged directly around the molten zone inside the airtight chamber; the single crystal may then, however, be contaminated by degassing of the coil, If, in addition, the single crystal has to be doped from the vapour phase, the dopant being supplied in the form of a vapour in a carrier gas, for example, argon, ionization phenomena may occur near the coil and disturb the doping. Moreover, the volume of the metal chamber has to be large in order to avoid coupling with the heating coil.

In order to obviate the aforesaid disadvantages, to improve the survey of the processes and to simplify at the same time the required, hermetically closed chamber it is preferred to arrange a transparent, airtight chamber 3,690,848 Patented Sept. 12, 1972 around the rod and the seed crystal, and the inductance coil is arranged aound the chamber. Such a method and device are described in French patent specification No. 1,415,880.

However, since the coupling between the coil and the rod must be increased in proportion to the increase of the diameters of the rod and of the single crystal to be formed, the turns of the coil have to be of minimum diameter. In the device disclosed in the aforesaid patent specification these turns cannot be arranged closely to the chamber, since friction and vibrations, if any, during displacement of the coil relative to the chamber may cause damage. It is therefore necessary to leave adequate space between the coil and the chamber.

On the other hand, in order to avoid some of the liquid zone sticking to the inner wall of the chamber, the chamber must have a fairly large diameter. The diameter of the chamber must be further increased because it is often necessary to rotate the rod and/or the single crystal inside the chamber.

Owing to the aforesaid requirements, in the event of large diameter rods, the difference between the diameters of the floating molten zone and of the coil traversed by the high-frequency current may be high, the inductive coupling may be slight, the inner portion may not be heated sufficiently and the heating may be irregular. Further in a radial direction, strong temperature gradients may occur with large diameter rods so that the solidliquid interfaces are strongly curved and crystal deficiencies are involved. With very large diameter rods, the axial portion of the rod may even remain in the unmolten state.

An object of the present invention is to mitigate the disadvantages of the above method.

A further object of the invention is to enable the production of semiconductor single crystals of high diameter and satisfying crystal quality, which may, if desired, be doped from the vapour phase.

According to the invention the method of producing a semiconductor single crystal by floating zone melting of a vertical rod of semiconductor material in a controllable atmosphere inside a tubular, hermetically sealed chamber, which rod is capable of performing at least one transition movement relative to a coil surrounding the rod and traversed by a high-frequency current is characterized in that the coil surrounds said chamber at the level of a narrowed portion thereof, the inner diameter of which is smaller than the diameter of the solid parts of the rod and/or of the growing single crystal.

The narrowed portion of the tubular chamber at the level of the coil permits the use of a minimum coil diameter so that the high-frequency coupling between the coil and the melting zone is improved. This improvement permits the production of single crystal rods of larger diameter, while maintaining a satisfactory crystal quality.

The contraction of the chamber involves the necessity of a corresponding contraction of the melting zone at the same level.

This liquid zone assumes such a shape owing to the optimum coupling between the coil having the smaller diameter as allowed in accordance with the invention and the narrow neck of this liquid zone, which narrow neck is obtained by repelling forces between the current through the coil and the Foucault currents in the molten zone. Heating is more uniform owing to this smaller diameter neck, since owing to the skin effect the heat is normally only produced at the surface of the melt and is not restricted to large diameters.

The rates of the translatory movements of the rod and the seed of the single crystal to be formed may be con trolled so that the desired dimensions of the single crystal are obtained, whilst heating can be regulated in known manner.

The high-frequency coil of smaller diameter permits of carrying out the process by a seed crystal of smaller diameter, whilst adequate inductive coupling is maintained. It is known that a reduction of the section of the seed crystal is conducive to the obtainment of a single crystal of improved quality.

The diameter of the non-narrowed portion of the tubular chamber, which is now no longer determined by the diameter of the turns of the heating coil, need no longer be minimized; it is also possible to use non-calibrated tubes and to cause the rod and the single crystal to rotate without the risk of friction against the inner wall of the tube.

Since the heating coil is stationary relative to the cham ber, the risk of vibrations and damage due to friction of a coil turn against the tube wall is avoided. The inductive coils may even be pressed tight against the outer wall of the chamber. When a single turn of the coil is used, the loop thus formed can be closed more narrowly so that the homogeneity of the heating is improved, because the connections to a single turn produce a slight non-uniformity of the high-frequency inductive field. This improvement also permits of obtaining larger diameters of the single crystals of good quality.

Since the heating coils are located outside the tubular chamber, the risk of ionization of the atmosphere inside the chamber is avoided as well as the risk of contamination of the single crystal by the material of the coil.

In a preferred form of the method according to the invention the rod is molten from the lower end, whilst the single crystal is given the diameter of the rod and the neck of the melting zone is given a diameter at the most equal to half the rod diameter. The single crystal connected with the seed can be set in a translation and a rotation movement the rod can be set in at least one translation movement,

The single crystal is usually obtained from a polycrystalline rod. In this case a seed crystal is provided at the lower end of the device, brought into contact with the lower end of the polycrystalline rod, which is melted, and the melting zone is displaced by the translatory movement of the seed crystal and of the polycrystalline rod. The translatory movements of the seed and the rod are preferably independent of each other.

The seed crystal has a diameter smaller than that of the neck of the tubular chamber at the level of the contraction. The seed and the polycrystal are rotated, preferably in opposite directions so that the melting zone is slightly stirred.

The device suitable for carrying out the method in accordance with the invention comprises preferably a quartz tube surrounded at the level of a narrowed portion by an inductive coil formed by a single turn, which is cooled by circulating water and is made, for example, of copper shaped around the narrowed portion of the chamber, the loop being closed as tightly as possible. The quartz tube comprises gas inlet and gas outlet tubes. The tube is sealed by sealing members having each a sealed throughconnection for the shafts to which the holders of the seed and the rod are secured. The device according to the invention comprises means for carrying out the translatory and rotary movements of the seed and the rod of starting material, which are adapted to operate independently of each other. It is preferred to arrange two rotatory and translatory heads opposite one another on a common support, to which the tube and the heating coil are secured.

As a matter of course, known refinements such as an additional heating, diameter control-members, heating correction means, control-members for the volume of the melting zone may be employed in the device embodying the invention. The invention is particularly suitable for the production of a single crystal having a diameter differing from that of the polycrystalline rod of starting 4 material. The invention permits an eccentric arrangement of the heating coil relative to the rod or an eccentric arrangement of the seed relative to the polycrystalline rod, Whilst the correct inductive couplings are maintained.

The invention will now be described more fully with reference to the accompanying drawing.

FIG. 1 is a schematic sectional view of a device embodying the invention.

FIG. 2 is a detailed view of that portion of the device shown in FIG. 1, which is located to the level of the melting zone.

Inside a transparent silica tube 1, sealed by two plugs 2 and 3, a seed crystal 4 is held in a holder 5 at the end of a shaft 6, which is passed in airtight manner through the plug 2, whilst a polycrystalline rod 7 is held in a holder 8 at the end of a shaft 9, which is passed in airtight manner through the plug 3.

The tube 1 is provided with a gas inlet tube 10 and a gas outlet tube 11; the tube has a narrowed portion 12; this portion accommodates a turn 13 formed by a copper tube, which is traversed by a high-frequency current and can be simultaneously cooled by circulating water.

In order to melt the zone in a rod 7 of a diameter of at least 38 mms. the tube 1 may have an inner diameter of 48 mms., which is reduced to 30 mms. at the level of the arrowed portion 12.

The device is shown in operation in a stage in which a portion of the rod 7 is melted and a seed or single crystal 14 is partly formed. A melting zone 15 is in a state of equilibrium between the rod 7 and the single crystal 14, the profile of this zone is determined by the surface tension of the liquid, its weight and by forces produced by interaction of the current through the turn 13 and the Foucault currents in the zone. The zone 15 thus has a bulging portion 16 and a narrowed portion 17. On the side of the starting or polycrystalline rod 7 the zone is bounded by the melting front 18 and on the side of the seed 14 by the solidification face 19. The zone 15 may have a diameter of 18 to 20 mms. in the narrowed portion and a diameter of at least 38 mms. at the rod 7.

The volume and the shape of the melting zone 15 are obtained by controlling the frequenty and the intensity of the current through the turn 13.

The seed 14 and the rod 7 are caused to rotate each in the direction of the arrows 20 and 21 respectively and to perform a translatory movement in the direction of the arrows 22 and 23 respectively.

The translatory and rotary moving rates are determined in accordance with the conventional criteria; the rotations have to determine the homogeneity of the melting phase and the regularity of the crystallisation front and the translatory movements determine the progression of the melting zone along the rod 7 and the diameter of the single crystal 14 relative to that of the rod 7.

From FIGS. 1 and 2 it will be apparent that the diameter of the heating turn 13 is considerably smaller than that of a coil arranged in known manner around a tube 1 having no narrowed portion and that the inductive coupling between the turn 13 and the zone 15 is at the optimum.

At the beginning the inductive coupling between the turn 13 and the seed 14 of small diameter is better than that of a coil of a fairly large diameter surrounding a tube 1 without narrowed portion.

The tube 1 is transparent so that the whole process can be satisfactorily observed. The single crystal may be doped from the vapour phase without the risk of ionization phenomena. The single crystal is protected from contamination from the heating coil.

As a matter of course, the portion of the tubular chamber on either side of the narrowed portion may have shapes differing from those shown in FIG. 1.

What is claimed is:

1. A method of producing a semiconductor single crystal solid rod by floating zone melting of a polycrystalline rod of semiconductor material in a controllable atmosphere which comprises:

enclosing said polycrystalline rod in a tubular hermetically closed chamber, having a pinched in portion;

supporting said polycrystalline rod from the top in substantially vertical orientation within said chamber, so that the lower end of said polycrystalline rod is disposed adjacent said pinchedin portion;

supporting a seed crystal from the bottom within said chamber;

flowing inert gas through said closed chamber as said controllable atmosphere;

establishing an inductively coupled field through said pinched portion to form a molten zone in said polycrystalline rod;

controlling said field so that said molten zone is maintained with a narrowed portion which is smaller than said pinched in portion of said closed chamber;

contacting said molten zone with said seed crystal;

rotating said seed crystal while withdrawing said seed crystal from said molten zone and simultaneously rotating said polycrystalline rod; and

moving said seed crystal and said polycrystalline rod downward so that said molten zone progresses along said polycrystalline rod while a single crystal solidification takes place below said molten zone, the rate of downward movement being such that said single crystal is formed with a diameter substantially equal to the diameter of said polycrystalline rod.

2. A method as claimed in claim 1, which includes maintaining said narrowed portion at a diameter which is no larger than one-half of the diameter of said polycrystalline rod.

3. An apparatus adapted to produce a semiconductor single crystal solid rod by floating zone melting of a polycrystalline rod of semiconductor material in a controllable atmosphere, comprising:

a transparent hermetically closed chamber having tubular walls and having a gas inlet and a gas outlet at opposite ends of said chamber, said tubular walls having a pinched in portion located between said gas inlet and said gas outlet;

at least one inductance coil disposed about said pinched in portion of said tubular walls; first means for introducing a polycrystalline rod into said chamber the said rod having a melt adjacent the pinched portion, the pinched portion and said adjacent portion of the melt being of smaller cross-sectional area than the rod; second means for rotating and axially translating said polycrystalline rod within said chamber and through said pinched in portion; third means for introducing a seed crystal into said chamber; and fourth means for rotating and axially translating said seed crystal within said chamber.

4. An apparatus as claimed in claim 3 wherein said first and third means are plugs which sealingly fit into opposite open ends of said chamber.

5. An apparatus as claimed in claim 4 wherein said inductance coil is hollow and is cooled with circulating fluid.

6. An apparatus as claimed in claim 3 wherein said second means and said fourth means rotate said polycrystalline rod and said seed crystal, respectively, independent of each other.

7. An apparatus as claimed in claim 6 wherein said second means and said fourth means are adapted to perform translatory movements which are independent of each other.

References Cited UNITED STATES PATENTS 2,927,008 3/1960 Shockley 148-l.6 3,241,924 3/1966 Karstensen 23-301 SP 3,260,573 7/1966 Ziegler 23-301 SP 3,259,468 7/1966 Steven et al. 23-273 SP 3,397,042 8/1968 Hunt 23-273 SP 3,249,406 5/1966 Crosby et al 23-273 SP FOREIGN PATENTS 1,022,790 3/1966 Great Britain. 1,187,625 4/1970 Great Britain.

NORMAN YUDKOFF, Primary Examiner S. SILVERBERG, Assistant Examiner U.S. Cl. X.R. 

